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Item Type:

Ph.D. Thesis

Title:

Exclusive eta Meson Reconstruction in Proton-Proton Collisions at 2.2 GeV with the HADES Spectrometer and High Resolution Tracking

Language:

English

Abstract:

HADES (High Acceptance DiElectron Spectrometer) is a unique apparatus currently assembled at heavy-ion synchrotron SIS at GSI, Darmstadt. The main part of the HADES physics program is focused on in-medium properties of the light vector mesons ρ(770), ω(783) and φ(1020), but also concerns several important problems of hadron physics like: validity of the Vector meson Dominance Model (VDM) in the description of meson and baryon Dalitz decays and of the electromagnetic form-factor of the nucleon in time-like region. In the framework of the present thesis a detailed study of the exclusive η meson reconstruction in proton-proton collisions at 2.2 GeV kinetic energy of the incident proton beam is presented. The exclusive measurement of the η mesons has been performed for two electromagnetic decay channels (η→ π+π-π0 and η→ e+e-γ), the relative branching ratios of which are known. In order to obtain better tracking resolution, an algorithm of kinematic fit has been developed and tested. The main goal of the kinematic fit is to improve the reconstruction resolution by using physical constraints governing the process. Studied results have been used to evaluate the dielectron detection efficiency of the HADES experiment. The kinematic fit algorithm has been used for above mentioned analysis as well. Apart from these studies, using the kinematics of the elastic scattering channel, the spectrometer performance has been studied in terms of reconstruction resolution and detector alignments. From a technical point of view, a new method for momentum reconstruction, using a cubic spline track model in the magnetic field region of the HADES spectrometer, has been developed. An algorithm for matching of different hits belonging to one track reconstructed by the spectrometer systems has also been developed. Both algorithms have been used for the data analysis described in this work. The phase-space distributions of η→ π+π-π0 and η→ e+e-γ decay channels being different, the relative branching ratio of these decay channels will differ from that obtained for full phase space. Therefore the simulation data has been generated in order to find out the relative branching ratios of the above mentioned decay channels inside the HADES acceptance. We have done different types of simulations in order to compare with the data. In particular, the anisotropy in the angular distributions of the η mesons, observed by the DISTO experiment, have been included in the simulation and compared with our data. According to the VDM, the photon interacts with the hadron of interest not only directly, but also through vector mesons. In particular for the η meson, which is a neutral pseudoscalar meson, the Dalitz decay goes in two steps, where in a first step it emits two photons (real plus virtual), and in a second step the virtual photon decays into a e+e- pair. The validity of the VDM model in describing the Dalitz decay of the η meson has been investigated by comparing the experimentally reconstructed invariant-mass distribution of the e+e- pairs with the VDM model predictions. To investigate the production mechanism of the η meson, their angular distributions in the center-of-mass system have been studied. This thesis is divided into two parts: In the first part a brief introduction to the physics program of the HADES experiment is given. A description of the detector systems of the spectrometer and the detailed explanation of track reconstruction algorithm of the HADES experiment is given in this part as well. The second part mainly concerns the analysis of the proton-proton data and the discussion of the results.